In papermaking machinery, determination of machine direction (MD) and cross-machine (CD) weight profiles is generally performed by scanning sensors that operate at the dry end of the paper machine. As a result, MD and CD weight control response time has been limited by the time necessary to obtain weight and moisture profiles from the scanning system. The time delay in getting the paper web to the scanning system is the total transit time through the paper machine plus the time required to thread the sheet from the wire into and through the press section, then the dryer, and finally through the calender. The accumulation of all these times often exceeds 30 minutes. It also takes an additional five minutes or so to acquire enough scans to give a reliable estimate of the MD/CD profiles. This accumulated time represents lost production time as paper manufactured during this period is discarded to the broke pit.
To address the above shortcoming of existing papermaking machinery, novel sensors have been developed to obtain rapid measurement of the MD/CD weight profile of the paper web as soon as possible after the start of the paper making process. The novel sensors are the subject of co-pending U.S. patent applications Ser. No. 08/766,864 and Ser. No. 08/789,086.
The faster the CD profile is measured and stabilized at start up, the easier the sheet threading procedure will be and the faster the time to saleable paper product with minimum loss to the broke pit. The novel sensors are positioned at the wet end of the papermaking machine at or near the dry line under the wire portion of a single wire of the machine. Alternatively, the sensors can be positioned adjacent a side of one wire of the two wires on a twin wire machine. The sensors operate by measuring the water and fiber weight on the wire portion. Each sensor comprises an array of sensor elements that measure the weight of the water on the wire by measuring the effective electrical properties of the water between individual sensor elements. The effective electrical properties are related to the water weight over each sensor element. The water weight in turn is correlated to the fiber weight. The sensor array is built into a foil-like structure that extends below the wire in the cross-machine direction. The sensor array is therefore able to simultaneously monitor the full CD width of the papermaking machine. Typically, up to 512 CD data boxes can be provided giving a 25 mm CD resolution. Full profile data can be measured at up to 600 times per second and this is used to calculate MD updates 10 times per second and CD control profiles every second.
It is important that the sensor elements reliably contact the underside of the forming wire to ensure that accurate measurements are taken. This has been a weakness of prior designs. It is necessary that the sensor arrays be removable to permit maintenance and replacement. Prior art mounting systems rely on a T-rail and T-slot arrangement to removably mount the sensor array to a base mounting beam. Relatively large clearances are required for installation and removal of such an arrangement which prevents a consistent mounting and alignment of the sensor array with respect to the wire with the result that data obtained from the sensors is not always reliable. Furthermore, the T rail/T slot arrangement limits the vertical position of the sensor array to the location of the T rail without the possibility of adjusting the sensors to ensure proper contact with the wire.